82 
ICE-PERIOD IN AMERICA. 
than in Norway, Great Britain, or Switzerland. 
In short, the ice of the great glacial period in 
America moved over the continent as one con¬ 
tinuous sheet, overriding nearly all the ine¬ 
qualities of the surface. Thus the peculiar 
physical character of the country gives a new 
aspect to the study of glacial evidences here. 
The polished surfaces stretch continuously 
over hundreds and hundreds of miles; the 
rectilinear scratches, grooves, and furrows are 
unbroken for great distances ; the drift spreads 
in one vast sheet over the whole land, con¬ 
sisting of an indiscriminate medley of clays, 
sands, gravels, pebbles, boulders of all dimen¬ 
sions, so uniformly mixed together that it 
presents hardly any difference in its composi¬ 
tion, whether we examine it in New England, 
New York, Pennsylvania, Ohio, Michigan, In¬ 
diana, Illinois, Wisconsin, in Iowa beyond the 
Mississippi, in the more northern Territories, 
or in Canada. 
In Europe, boulders of large dimensions do 
not often occur within the drift, but are 
usually resting above it with their sharp angles 
and rough surfaces unchanged, having trav¬ 
elled evidently upon the glacier and not under 
